A test and measurement instrument, such as an oscilloscope, generally uses a trigger to mark a specific point in time in an input signal, and to cause the instrument's acquisition system to acquire the input signal data, thereby defining the specific portion of the input signal that will be stored in the instrument's memory. The trigger serves to synchronize the horizontal sweep of the instrument, allowing the instrument to show a stable depiction of the input signal on a display. The instrument may trigger on the same input signal that is shown on screen, or it may trigger on a different trigger source, such as another input signal, an external trigger signal, or even the line voltage signal. Conventional test and measurement instruments are capable of triggering on a wide variety of conditions that may be present in a trigger source signal. For example, all modern oscilloscopes include an edge trigger mode, which generates a trigger when the trigger source signal experiences an edge transition.
Many modern test and measurement instruments also include a logic trigger mode. The logic trigger mode is especially useful for verifying or troubleshooting the operation of digital logic. In the logic trigger mode, the instrument triggers on a logical combination (e.g. NOT, AND, OR, NAND, NOR) of Boolean words in input signals. For example, in an oscilloscope with two input channels (e.g. CH1, CH2), the oscilloscope in logic trigger mode may be set to trigger when, according to Boolean logic, CH1 is logically HI, OR when CH2 is logically HI. The specific voltage levels used by the instrument to determine when an input signal is logically HI or LO are generally selectable in the instrument. Most instruments contain preset levels corresponding to particular logic families (e.g. TTL, ECL, CMOS).
The behavior of the conventional logic trigger circuit just described, particularly the logical OR trigger mode, is correct according to Boolean logic, but is nevertheless limited in what types of input signal transitions it can recognize, and is unsatisfactory for some test and measurement instrument users. Embodiments of the present invention address these and other limitations of the prior art.